As is well known, a printing machine has a plurality of successive plate cylinders each of which carries a printing plate. Typically, each printing plate is used to print a different primary color so that as a sheet of paper has superimposed images transferred to it by the succession of plates, a multi-color image is produced. In order to produce a sharp multi-color image, the successive plate cylinders must be positioned with respect to each other so that the plates are in a predetermined relative alignment (i.e., in register).
Arrangements for adjusting the register of plate cylinders have been previously described in various publications (e.g., DE-PS 3,136,703, DE-OS 3,136,704, DE-OS 3,302,200, DE-OS 3,535,579 and JP-PS 55-25062/1980). Generally, these known arrangements comprise a reflex register marker detection system for each plate cylinder, i.e., a light source, reflecting markers (e.g., line markers in the form of registration crosses or stamped-out apertures on the printing plates) and a photo-receiver, with the light source and receiver being positioned on the same side of the printed sheet to illuminate the marker by means of the source and to detect reflected light by the receiver. Each system delivers a measurement signal scanned by the reflex scanning system or the like from the reflecting markers, and the measurement signal is used to generate control signals for adjusting the register of the plate cylinder. Such reflex scanning systems are acceptable, but their accuracy depends greatly on the quality of the marker edges and contrast since the reflecting markers are scanned in the form of shadows in the bright surrounding field of the photo-receiver.